Apparatus for detecting the condition of a sheet or web

Abstract

For the detection of defects such as tears or fold lines in banknotes, for example, continuous wave ultrasonic energy is directed by a transmitter to a flow path for the notes and ultrasonic energy reflected from the notes or transmitted through tears or holes in the notes is converted by a receiver into an electric signal. A signal analyzing circuit is responsive to changes in the output of the receiver during the passage of the banknote. In the ultrasonic inspection area, the note may be passed over rollers with flared ends to cause a tear to open and allow the passage of ultrasonic energy. In a reflective system the note may be passed over a roller to cause one or both of the adjacent edges of a tear to leave the roller surface. The electric signal is analyzed for components indicative of a defect; for tear detection, a spike filter and a circuit for distinguishing tear signals from end-of-note signals may be used. For crinkle detection, the received signal may pass through a differentiator, an integrator and a threshold comparator.

I claim: 
     
       1. In apparatus for detecting defects in webs or sheets, and having means defining a flow path for the web or for the sheets to be inspected, the combination of: an ultrasonic transmitter arranged to direct ultrasonic energy towards the flow path in an inspection area;   an ultrasonic receiver directed to receive ultrasonic energy from the inspection area;   and signal analysing means (FIG. 7) responsive to the output of the receiver;   the transmitter directing the ultrasonic energy towards the inspection area as a continuous wave and the signal analysing means providing an output signal in response to changes in the output of the receiver during the passage of the web or sheet indicative of a defect in the inspected web or sheet;   and wherein the receiver includes means defining in the inspection area a flow path which is convex in the longitudinal direction of the flow path, whereby the sheet or web is caused to assume a convex bend in the area in which the ultrasonic energy impinges upon it.   
     
     
       2. Apparatus in accordance with claim 1, in which the ultrasonic transmitter is arranged on one side of the flow path and the ultrasonic receiver is arranged on the other side of the flow path, and comprising in the vicinity of the inspection area a roller over which the sheet or web is passed, the roller having ends shaped to cause the marginal portion of a web or sheet to be deflected out of the plane of its central portion, the change in the cross-sectional form of the sheet or web causing marginal tears to open allowing ultrasonic energy from the transmitter to pass through the tears to the receiver. 
     
     
       3. Apparatus in accordance with claim 2, in which the roller has end portions which increase in diameter towards the ends. 
     
     
       4. Apparatus according to claim 2, comprising two rollers with similarly shaped ends, spaced along the flow path, the flow path passing over one roller and then under the next, whereby a tear in the sheet or web passing along the flow path opens in the space between the two shaped rollers. 
     
     
       5. Apparatus in accordance with claim 1, in which the receiver is arranged to receive ultrasonic energy reflected from the sheet or web in the inspection area. 
     
     
       6. Apparatus in accordance with claim 1, comprising a roller defining the flow path in the inspection area, the sheet or web passing over the roller. 
     
     
       7. Apparatus in accordance with claim 6, in which the roller is formed with flared ends, whereby the effect of folds in a sheet or web on the received signal is reduced but the effect of tears is not reduced. 
     
     
       8. Apparatus in accordance with claim 1, comprising a belt system extending along the central portion of the flow path for feeding the sheet or web along the flow path. 
     
     
       9. Apparatus in accordance with claim 1, comprising a circuit receiving electric signals from the ultrasonic receiver, means for detecting abrupt changes in the signal such as are caused by the edge of a sheet or a tear in the sheet, and means for counting the abrupt changes during the passage of a sheet to ascertain whether the number of such changes indicates at least one such change indicative of a defect between the leading and trailing edges of the sheet. 
     
     
       10. Apparatus in accordance with claim 1, for detecting crinkle in a sheet, in which the signal analyzing means further comprises a demodulator receiving an electric signal derived from the ultrasonic receiver, a differentiator for deriving signals related to the positions of the crinkled portions of the note, an integrator connected to receive the signals from the differentiator, and a comparator comparing the integrator output with a preset or programmed crinkle level.